Over on the Capcom*Unity forums, fans continue to exist in torment as Capcom's Senior Community Manager, Seth "S-Kill" Killian continues to withhold his knowledge of one last mysterious secret which has yet to be discovered in Mega Man 9.
After a number of tasteless taunts and insults in the Ask Capcom thread dedicated to the subject, Seth has finally broken his silence to give the following information about it:

Hi guys,

Trike wrote me a perfectly reasonable PM, to which I'll reply here. As I said before, I am honestly not trying to torture anyone with this.

I made an off-hand comment about an undiscovered MM9 surprise. As predicted, nobody has found it yet. I certainly don't intend for anyone to kill themselves looking for something--I am quite positive it WILL eventually be found some day, at which point I'm sure everyone will complain bitterly.

The Gouken/Akuma reveal is a bit different--that's core functionality that everyone already knew about, and was unlocking on their own, they just weren't sure about the specifics. The MM9 surprise is nothing like unlocking some core functionality in the game--it's just a little surprise, and you can enjoy the whole game without ever knowing anything about it.

I am not planning on spilling the beans (though the personal attacks really motivate me!), and although this might make no difference, please try not to let it frustrate you--if it's that important to you, just be happy in the thought there might still be a nugget more to learn about a favorite game (there is certainly always more to learn about Street Fighter). It is a nugget, not some cornucopia of alt outfits that powerup a screen-filling Black Hole Bomb.

best,

Seth

So then, that just leaves this one simple question: Have you discovered the secret of Mega Man 9? If so, let us know, and we'll sing your praises from the rooftops.